Last week, we saw how Hume made a strong case against himself in the Dialogues concerning Natural Religion. The self-directed charge is that those who claim that religious belief is not supported by reason and evidence are guilty of "an unequal conduct" which is "plain proof of prejudice and passion". The same sorts of reason and evidence which are deemed inadequate to support religion are used to support all the other beliefs they have in the material world. They are happy for human reasoning to lead them to belief in invisible and unseen entities and events like dark matter and the big bang. When others use the same tools and are led to invisible and unseen divine entities, however, they claim human understanding is not fit for this purpose.

Is this not hypocrisy? Hume's rejoinder, delivered through the mouth of Philo, is both subtle and plain. It relies on no dubious, clever intellectual manoeuvres, but nor is it so obvious that everyone would leap to agree with it.

At the heart of his argument is his idea of mitigated scepticism. On the one hand, he thinks that pure reason, far from giving us sufficient grounds for the truth of our beliefs, actually undermines any certainty we might have about them.

If reason be considered in an abstract view, it furnishes invincible arguments against itself; and that we could never retain any conviction or assurance, on any subject.

However, no one can genuinely believe in or live by this kind of thoroughgoing scepticism for one moment. Sceptical reason therefore requires a "counterpoise", in the form of "the more solid and more natural arguments derived from the senses and experience." In other words, we have to take some things as given, such as the existence of an external world, and the fundamental principles of consistent thought, even though we can provide no ultimate, robust rational foundation for them.

In combining sceptical reason and knowledge from experience, "The one has no more weight than the other. The mind must remain in suspense between them."

The clearest example Hume ever gave of this balancing act came in the Enquiry Concerning Human Understanding, in the oft-cited but rarely understood discussion of cause and effect. The conflict between reason and experience is here stark. There is simply no rational basis for the belief that every event has a cause, or that there is any kind of necessary connection between what we call causes and their effects.

However, to say that Hume was therefore a sceptic about causation is grossly misleading. In fact, he argued that none of our experience makes sense without the assumption that causation is real. The idea of necessary connection, he says, "is every moment necessary for us to treat in all our disquisitions." On it "are founded all our reasonings concerning matters of fact or existence." In other words, even the simplest, most obvious reasoning about matters of fact has to take as basic a belief in cause and effect that reason itself cannot ground.

Science, however detailed it gets, is based on the same proper balance of reason and experience. It simply takes the observable regularities of nature, upon which all reasoning depends, and gives ever more systematic accounts of them. "To philosophise on such subjects," Philo says, "is nothing essentially different from reasoning on common life."

"Natural religion", however, fails to maintain this balance, and falls off the metaphysical tight-rope. "When we look beyond human affairs and the properties of the surrounding bodies," says Philo, in just part of one of Hume's notoriously long sentences, "…we have here got quite beyond the reach of our faculties."

We are like foreigners in a strange country, to whom every thing must seem suspicious, and who are in danger every moment of transgressing against the laws and customs of the people with whom they live and converse. We know not how far we ought to trust our vulgar methods of reasoning in such a subject; since, even in common life, and in that province which is peculiarly appropriated to them, we cannot account for them, and are entirely guided by a kind of instinct or necessity in employing them.

Whereas science explains the observable by studying the actual behaviour of what is observed, natural religion explains the unobservable by analogy with what is observed. Philo explains this key difference in Part II of the Dialogues:

That a stone will fall, that fire will burn, that the earth has solidity, we have observed a thousand and a thousand times; and when any new instance of this nature is presented, we draw without hesitation the accustomed inference. The exact similarity of the cases gives us a perfect assurance of a similar event; and a stronger evidence is never desired nor sought after. But wherever you depart, in the least, from the similarity of the cases, you diminish proportionably the evidence; and may at last bring it to a very weak analogy, which is confessedly liable to error and uncertainty.

The difference is vital, and it explains why greater scepticism about the power of reason to establish religious truth is justified than for the power of reason to establish scientific, historical, and other truths about the physical world.

Two points are worth stressing about Hume's argument here. The first is that it is not an argument against all religion, only the attempt to establish religion as a reasonable hypothesis based on observation of the universe.

The second is that Hume's scepticism, though "mitigated" is very real. Hume is often held up as a hero of the enlightenment, and so he should be. But he had just about as pessimistic a view about the power of reason it is possible to have without abandoning it as altogether useless. In contrast, some self-proclaimed modern heirs of the enlightenment talk as though science and reason were two invincible superheroes, capable of leading us to certain truth. Hume is a challenge to this complacency, as well as to the beliefs of many religious believers.

For Julian's previous blogs on David Hume and religion, visit the How to believe series page